Typical magneto-optical (MO) disc drives record data by locally heating a portion of the disc. MO discs, or media, include a recording layer of a magnetic material. The coercivity of the heated portion of the media is lowered when it is heated by the laser beam. This allows the magnetic polarity in that area to be reversed by an applied magnetic field. In such disc drives, data is read from media by illuminating areas of the storage media with a linearly polarized laser beam. The Kerr rotation effect causes the plane of polarization of the illuminating beam to be rotated. The direction of rotation depends on the magnetic polarity in the illuminated area of the storage media. When the disc is read, the polarization rotation is determined with a pair of optical detectors and a polarization beam splitter to produce an output data signal. Limitations of MO disc drives include data access time and density with which data can be stored.
FIG. 1 is a diagram of prior MO recording system 100 typically used with 130 millimeter (mm) diameter MO media. System 100 is an example of a "substrate incident" recording system. In substrate incident systems, laser light is incident on a thick substrate, travels through the substrate, and is focused on a recording layer below the substrate. System 100 includes objective lens 102 for focusing a collimated beam of light on disc 116. Disc 116 is an example of a typical two-sided MO disc. MO disc 116 includes substrates 104 and 114 forming outside layers on opposing sides of disc 116. Substrates of 104 and 114 are made of materials such as plastic polycarbonate and are approximately 1.2 mm thick. Recording layer 106 is below substrate 104, and recording layer 112 is below substrate 114. Recording layers 106 and 112 can be made out of any one of a number of well-known materials, such as Tb--Fe--Co, a rare-earth transition-metal alloy. The laser light beam passing through objective lens 102 penetrates substrate 104 as shown and is incident on a focal point on the surface of recording layer 106.
System 100 has several disadvantages. One of the disadvantages of system 100 is that it is necessary to apply energy to the recording layer to erase data prior to writing new data. This is because a large, stationary magnetic coil (not shown) having a large inductance is situated on the opposite side of disc 116 from objective lens 102 to assist in the writing process. Because the coil is held at a relatively great distance form the media surface and has a relatively large inductance, the magnetic field cannot be reversed at high frequencies. Therefore, it is necessary to erase data before writing new data. The necessity of erasing before rewriting slows the process of writing data to disc 116.
Another disadvantage of system 100 is that the density of data stored on disc 116 is relatively low. A further disadvantage of system 100 is that only one side of disc 116 can be accessed at one time. This is because the relatively large coil occupies the space on the side of the disc opposite the objective lens. This space cannot therefore be used for another lens and actuator. In order to access a different side of disc 116, disc 116 must be removed, turned over, and reinserted into system 100. Disc 116, however, provides good data security because relatively thick substrates 104 and 114 allow disc 116 to be handled without danger of data loss or difficulty in reading data because of contamination. The thickness of substrates 104 and 114 presents difficulties, however. Because the substrate material is applied so thickly, non-uniform heating and cooling characteristics at the outer radii cause non-uniform solidification during the molding process. Therefore, light passing through the substrate at the outer and inner radii is distorted such that birefringence occurs. The edges of the disc are thus unsuitable for data storage, and the data storage capacity of the disc is reduced. In addition, typically, with a thick substrate, some degree of light distortion is experienced throughout the disc.
Another disadvantage of system 100 is its susceptibility to tilt. Tilt is the deviation from 90 degrees of the angle between the axis of incident light and the plane of the recording layer of the media. Because incident light must pass through a relatively thick substrate before reaching a recording layer, the incident light becomes distorted with the occurrence of tilt. This effectively limits the possibility of reducing the spot size of the incident light, and consequentially effectively limits the possibility of increasing the storage density of the MO disc. It is desirable to decrease the spot size because a smaller spot size corresponds to a larger numerical aperture (NA) of the system. There are factors that place an upper limit on NA. One of the factors is tilt. Therefore, in a system such as system 100, susceptibility to tilt limits the increase of NA and limits storage capacity of the disc.
FIG. 2 is a diagram of another prior MO recording system 200. Collimated light beam 202 passes through objective lens 204 to disc 216. Disc 216 includes substrate 206 that is typically 0.6-1.2 mm thick. Disc 216 further includes recording layer 208 between substrate 206 and protective layer 210. In system 200, the large, stationary coil of system 100 is replaced by a relatively small coil in flying magnetic recording head 214. Flying height 212 is maintained by an air bearing created when disc 216 passes under flying magnetic recording head 214. For writing to disc 216, a magnetic field created by magnetic recording head 214 is used in conjunction with collimated light 202 which passes through objective lens 204. The smaller coil of magnetic recording head 214 has less inductance than the large, stationary coil of system 100. The reduced inductance allows direct overwrite of data on disc 216 by switching the magnetic field.
System 200 still possesses the disadvantage of relatively low storage densities. For example, because system 200 is a substrate incident system, the thick substrate 216 causes system 200 to have the same susceptibility to tilt as system 100. In addition, disc 216 is a one-sided, rather than a two-sided disc, which reduces overall storage capacity.
System 200 also has the disadvantage of requiring mechanical coupling of light on one side of disc 216 with magnetic recording head 214 on the other side of disc 216. Typically, this coupling is accomplished by mechanical linkages that pass from objective lens 202 to magnetic recording head 214 around the edge of disc 216. The mechanical linkages cannot be allowed to interfere with the movement of objective lens 202 or with disc 216.
FIG. 3 is a diagram of prior MO recording system 300. System 300 is an example of an "air incident" design in which a lens is held very close to the media and laser light is incident on very thin protective layer 309 that is over recording layer 308 of disc 318. Other prior art air incident designs may have no protective layer, wherein a surface of recording layer is the surface of the disc. System 300 employs flying magnetic recording head 316, and a two-piece objective lens comprised of lens 314 and lens 312. Prior art systems similar to system 300 use other lens designs, for example, single lens or three-piece objective lens designs. Lens 314 is held extremely close to disc 318. Collimated light beam 302 passes through lens 312 and lens 314. Lens 312 and lens 314 are integrated with slider 304 and magnetic recording head 316. Flying height 306 for system 300 is typically less than the wavelength of the laser light used in reading from and writing to MO disc 318.
Disc 318 has an MO recording layer 308 over substrate 310. Because in system 300, flying objective lens 314 is in close proximity to disc 318, the need for a focus actuator is eliminated. As is known, focus actuators are mechanisms that adjust the height of an objective lens over a disc during read and write operations. In the case of system 300, the height of flying lenses 312 and 314, and thus the focus of flying objective lenses 312 and 314, are determined by the distance between recording layer 308 and flying objective lenses 312 and 314 as determined by the air bearing created between slider 304 and disc 318 during flight.
By maintaining the spacing between flying objective lenses 312 and 314 and recording layer 308 at less than the wavelength of the laser light used, laser light can be focused in the near field mode of operation. As is known, the near field mode of operation uses the phenomenon of evanescent coupling, which requires that the objective lens be held very close to the recording layer. The use of evanescent coupling to perform recording allows a smaller spot size, and therefore, greater recording densities and better data throughput.
System 300 has several disadvantages. For example, the surface of very thin protective layer 309 or the surface of lens 314 closest to the disc can be contaminated, causing permanent damage to data or to the disc drive system.
Another disadvantage of system 300 stems from the fact that because there is one objective lens and no focus actuator, the flying height must be tightly controlled to adequately control focus. Variations in the flying height and thickness of very thin protective layer 309 over the recording layer must be controlled within the depth of focus tolerance of the flying lens. Generally, the tolerance of flying height 306 and protective layer 309 thickness is a percentage of the nominal thickness. Therefore, in order to reduce the tolerance, the nominal thickness of very thin protective layer 309 must be reduced. For example, the depth of focus tolerance is generally plus or minus 0.5 micron. A typical tolerance in applying very thin protective layer 309 is ten percent of the thickness of the protective layer. Therefore, flying height 306 and the thickness of very thin protective layer 309 together must be very small for the thickness variation of very thin protective layer 309 to be less than 0.5 micron.
In the case of a near field system such as system 300, the flying height (the distance between the bottom surface of flying lens 314 and the surface of recording layer 308) must be less than the wavelength of the laser light. The wavelength of the laser light is typically 700 nanometers. Therefore, the thickness of a protective layer on recording layer 308 would have to be on the order of 25 nanometers. This is extremely thin and would not protect data on recording layer 308 from manual handling in a removable disc application, or from corrosion or contamination during shelf life. Even with the protection of a cartridge that covers disc 318, some contamination from particles in the atmosphere or from humidity or corrosive gases is inevitable over time.
Another disadvantage of system 300 is that this system may require lubrication of the surface of the magnetic media because the extremely low flying height creates a possibility of contact between the media and the head. A lubricant is used to minimize friction on contact and possibly to prevent "head crash" onto the media. Providing the lubricant is an expense and the application of the lubricant is an additional step in the manufacturing process. The presence of the lubricant is also a disadvantage because it does not allow the media to be wiped while in the drive mechanism for the purpose of removing contaminants from the surface.
Conventional disc drives all share similar disadvantages related to access of data on a storage disc. Current disc drives, even those designed to access two-sided media, are limited to accessing one side of the media at a time. It has not been possible, previously, to simultaneously and independently access both sides of a two-sided disc. One of the reasons for this is that reading/writing head mechanisms on either side of the disc are constructed to move together or not at all. Current disc drives therefore have limited data access speeds. This disadvantage is shared by previous MO drives and drives using other technologies, such as those used in computer hard disc drives.
Technology exists to make multiple disc drives appear to a client device as a single drive. Redundant arrays of independent drives (RAIDs) divide incoming data into multiple streams which are written to multiple drives simultaneously. RAIDs can be used to increase throughput by dividing a single incoming data stream and writing portions of it to multiple drives simultaneously. RAIDs can also be used to achieve data redundancy by sending different copies of the same data to multiple drives simultaneously. Although access speed can be increased by using RAIDs, RAIDs are expensive and complex because they are merely devices containing duplicate conventional disc drives, each of which has all the limitations previously discussed with respect to current disc drives.